Recently, with the development of laser processing machines, magneto-optical devices utilizing the interaction of light and magnetism have become of much interest. One of these devices is an isolator, which operates as follows: When the light oscillated from a laser source is reflected by the optical system in its path and is returned to the light source, then it disturbs the light oscillated from the laser source thereby providing an unstable oscillation state; and the isolator prevents the phenomenon. Accordingly, based on the action, the optical isolator is arranged between a laser source and an optical member and is utilized therebetween.
The optical isolator comprises three parts, a Faraday rotator, a polarizer arranged on the light-incoming side of the Faraday rotator, and an analyzer arranged on the light-outgoing side of the Faraday rotator. The optical isolator functions based on its property that when light comes in the Faraday rotator thereof under the condition where a magnetic field is applied to the Faraday rotator in the direction parallel to the light running direction, then the plane of polarization rotates in the Faraday rotator. That is the Faraday effect. Specifically, the light of the incident light having the same plane of polarization as that of the polarizer is, after having passed through the polarizer, introduced into the Faraday rotator. The light is rotated by +45 degrees relative to the light running direction in the Faraday rotator, and then goes out of the isolator.
As opposed to this, when the light returning into the Faraday rotator in the direction opposite to the incident direction first passes through the analyzer, only the component of the light having the same plane of polarization as that of the analyzer passes through the analyzer and is introduced into the Faraday rotator. Then, in the Faraday rotator, the plane of polarization of the returning light is further rotated by +45 degrees in additional to the initial +45 degrees, and therefore, the plane of polarization thereof is at a right-angle of +90 degrees with respect to the polarizer, and the returning light cannot pass through the polarizer.
It is necessary that the material to be used for the Faraday rotator of the optical isolator mentioned above has a large Faraday effect and has high transmittance at the wavelength at which it is being used.
Recently, as laser processing machines, many devices with fiber laser have become much utilized. The oscillation wavelength of the laser is 0.9 to 1.1 μm, and terbium gallium garnet single crystal (abbreviation: TGG), terbium aluminum garnet single crystal (abbreviation: TAG), etc. are used as the material having a large Faraday effect and high transmittance at the wavelength (See Patent Document 1).
The Faraday rotation angle θ is represented by Formula (A):θ=V×H×L  (A)
In Formula (A), V is a Verdet constant, which is a constant determined by the material of the Faraday rotator; H is the intensity of the magnetic field; and L is the length of the Faraday rotator. For use as an optical isolator, L is determined so that θ=45 degrees.
Accordingly, the factors which determine the size of the optical isolator include the Verdet constant and the intensity of the magnetic field. The Verdet constant of terbium gallium garnet single crystal is 0.13 min/(Oe·cm), the Verdet constant of terbium aluminum garnet single crystal is 0.14 min/(Oe·cm). In case where a single crystal of the type is used and when the level of the magnetic field is 10,000 Oe, then it is necessary that the length of the Faraday rotator is 20 to 25 mm in order to rotate the plane of polarization of the incident light by +45 degrees. Accordingly, the Faraday rotator having that size must be used and a polarizer and an analyzer formed of, for example, a rutile crystal must be fitted to both sides of the Faraday rotator, or that is, the size of the optical isolator will have to be at least about 70 mm. For downsizing the module of fiber laser, the optical isolator must be downsized, and therefore, a material capable of shortening its constitutive member, the Faraday rotator, must be developed.
On the other hand, as a material having a large Faraday rotation angle per unit length, there is known iron (Fe)-containing yttrium iron garnet (commonly known as YIG) single crystal (see Patent Document 2). However, this material has a large light absorption at a wavelength of 0.9 μm and the absorption has some influence on wavelengths in a range of 0.9 to 1.1 μm. Therefore, this material is unsuitable for use in that range.    (Patent Document 1) JP-A-7-089797 (JP-A denotes a Japanese unexamined patent publication application)    (Patent Document 2) JP-A-2000-266947